1. Field of the Invention
This invention relates to cathode structures for cathode ray tubes, and to a method for producing them, and more particularly relates to multilayer cathode structures produced from laminates of at least two self-supporting layers, each layer having essentially the same composition.
2. Prior Art
Cathode structures for cathode ray tubes desirably exhibit uniform electron emissions over an extended life cycle and under a variety of operating conditions. In addition, such cathode structures may be manufactured at the lowest possible cost. Because of such stringent requirements, particularly reliability and cost, there is great reluctance on the part of high volume manufacturers of cathode ray tubes to introduce new cathode structures or methods. Nevertheless, presently used cathode structures and methods exhibit limitations sufficiently troublesome to justify continuing investigations of alternate structures and methods.
These limitations include poor adherence of the emissive layer to its substrate, non-uniformity of emissions, and variations in the cathode-to-grid spacings (K-G.sub.1 spacings) of the electron gun, resulting in out-of-specification values for cut-off voltages. Both non-uniformity of emissions and variations in the K-G.sub.1 spacings can result from non-uniformity in the thickness of the emissive layer. Particularly in the case of the sprayed coatings widely in use today, such non-uniform thicknesses occur not only from one cathode coating to another, but also within a single cathode coating. Sprayed coatings tend to lack not only the thickness uniformity, but also the degree of surface smoothness of coatings produced by other techniques, such as casting a film of the potentially emissive material in an organic binder matrix. See U.S. Pat. Nos. 2,974,364; 2,986,671; and 3,223,569, assigned to the present assignee. Such variations in thickness and surface smoothness can lead to variations in quality of the spot produced from the impingement of the electron beam on the phosphor screen.
Where the thickness of the cathode emissive layer desired is greater than that achieved by a single coating operation, such as spraying or casting of a self-supporting film, multiple applications are called for, thus compounding the complexity of the operation, and adversely affecting the reliability and cost of the resultant structure.
U.S. Pat. Nos. 4,197,152 and 4,197,153, assigned to the present assignee, describe the formation of cathode structures by attaching self-supporting emissive films to the cathode support, such films centered and adhered to the support by a solvent drop thereon. However, such films are currently less satisfactory in thicknesses in excess of about 0.003 inches, due to limitations in the process of forming them.
Accordingly, objectives of the present invention include: providing a cathode structure for cathode ray tubes with an emissive layer which exhibits both uniformity of thickness and surface smoothness; providing such a structure with an emissive layer which exhibits an overall thickness greater than that achieved by single coating operations; and providing a method for producing such structures simply and reliably.